A variety of applications exist with a need to feed a single reflector antenna to operate across multiple sub-bands disposed within a bandwidth. Typically such sub-bands are relatively narrowband. For example, many NASA airborne and space science applications have to support multiple electromagnetic sensor instruments that operate through the same shared reflector apertures. The applications may involve, but are not limited to measurements of aerosols, clouds, precipitation, snow water equivalent and wind velocities. Such instruments can include radiometers, active radar devices and scatterometers, and even can be combined with a communication link. Alternatively, the same aperture sharing approach can be used for multiband communication and so on.
Feeds of shared reflectors can be made using a number of horn antennas, viz. one horn for each sub-band. However, only one horn can be in the reflector focus for optimal illumination of the reflector surface. The remaining horns will be off focus and, thus, cannot provide optimal illumination of the reflector surface. Furthermore, the remaining horns may introduce blockage of the reflector. Alternatively, antennas comprising stacked patches using multi-layer circuit boards may also be designed to perform similar functions as reflector feeds; however, the phase center normal to the patch surfaces of such antennas differ depending on which patch is radiating, which may change depending on the frequency bands of operation. The proposed antenna does not suffer from such detuning of the reflector antenna optics over frequency.
Another approach is to employ a broadband array that allows operation on multiple sub-bands with an optimal reflector excitation, because the array feed can be installed in the focus. However, using a broadband array to feed a reflector is not straightforward, because such arrays can operate truly in broadband mode only if they are (1) electrically large and (2) fully excited. A typical array used to feed reflectors can be small to avoid blockage of the reflector. At the same time, small arrays may suffer from edge truncation and severe impedance mismatching. Another factor degrading impedance matching of feed arrays is fragmented excitation, when only a part of array is selectively used to drive particular bands of interest and, thus, those arrays are not fully excited.